Electric Drive System and Energy Management Method

ABSTRACT

An electric drive system includes energy storage system (ESS), a power conversion system, and an alternating cumin (AC) traction system. The ESS provides or receives electric power. The ESS includes a first energy storage unit and a second energy storage unit. The power conversion system is electrically coupled to the ESS for converting an input power to an output power. The AC traction system is electrically coupled to the power conversion system for converting the output power of the power convention system to mechanical torques. The AC traction system includes a first AC drive device and a second AC drive device. An energy management system (EMS) is in electrical communication with the ESS, the AC traction system, and the power conversion system for providing control signals.

BACKGROUND

Embodiments of the disclosure relate generally to electric drive systemsand energy management methods, useful in a variety of propulsion systemssuch as an electric vehicle (EV) system.

With the increased concern about energy crisis and environmentalpollution caused by the fossil fuel exhaust, there is a growing interestin developing electric-powered vehicles, in which conventional fossilenergy may be replaced by energy sources such as lead-acid batteries,fuel cells, flywheel batteries, etc. Electric drive system is one of thekey components in the electric-powered vehicles. In the electric drivesystem, several kinds of AC motors such as induction motor (IM),interior permanent motor (IPM) and switched reluctance motor (SRC) arewidely used for converting electric power into mechanical torques. It isdesired that the propulsion system may have features of high efficiency,and high performance with low cost. These features may be achieved bychoosing appropriate energy sources, appropriate AC motors, and thencombining them together in an appropriate structure.

Currently, most of the electric drive systems include a single energystorage system (ESS) or a single motor, or a dual ESS with a singlemotor. However, on the one hand, a single ESS may not satisfy both theenergy and power requirements such as low energy, slow charging rate,and a shorter life due to its own limitation of charge/dischargecharacteristics. On the other hand, a single motor may not work with ahigh efficiency across the entire operating range. It will causeproblems of oversize, high cost, and low efficiency in order to meethigh performances such as a short acceleration time and a long runningmileage of the propulsion system.

Therefore, it is desirable to provide systems and methods to address theabove-mentioned problems.

BRIEF DESCRIPTION

In accordance with one embodiment disclosed herein, an electric drivesystem is provided. The electric drive system includes an energy storagesystem (ESS), a power conversion system, an AC traction system, and anenergy management system (EMS). The ESS includes a first energy storageunit for providing or receiving a first power and a second energystorage unit for providing or receiving a second power. The powerconversion system is eclectically coupled to the ESS for converting aninput power to an output power. The power conversion system includes apower conversion device. The power conversion device includes a firstterminal coupled to the first energy storage Unit, a second terminalcoupled to the second energy storage unit, a third terminal, and afourth terminal. The AC traction system is electrically coupled to thepower conversion system for converting the output power of the powerconversion system to mechanical torques. The AC traction system includesa first AC drive device and a second AC drive device. The first AC drivedevice is coupled to the third terminal of the power conversion devicefor providing or receiving a first mechanical torque. The second ACdrive device is coupled to the fourth terminal of the power conversiondevice for providing or receiving a second mechanical torque. The EMS isin communication with the ESS, the AC traction system, and the powerconversion system for providing control signals.

In accordance with another embodiment disclosed herein, an energymanagement method is provided. The energy management method includesenabling at least one of the first AC drive device and the second ACdrive device based at least in part on an electric drive system speedsignal. The energy management method includes enabling at least one of afirst energy storage unit and a second energy storage unit. The energymanagement method includes sending power control signals to the powerconversion system for controlling power flow paths in the electric drivesystem. The controlling of the power flow paths includes receiving orproviding an input power by at least one of a first terminal and asecond terminal of the power conversion system and providing orreceiving an output power by at least one of a third terminal and afourth terminal of the power conversion system.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a block diagram of an electric drive system applied in apropulsion system for driving two loads in accordance with an exemplaryembodiment of the present disclosure;

FIG. 2 is a block diagram of an electric drive system applied in apropulsion system for driving a single load in accordance with anexemplary embodiment of the present disclosure;

FIG. 3 is a diagram of an electric drive system applied in an electricvehicle (EV) system in accordance with an exemplary embodiment of thepresent disclosure;

FIG. 4 is a diagram of an electric drive system with two separate powerconversion devices coupled in parallel applied in an EV system inaccordance with an exemplary embodiment of the present disclosure;

FIG. 5 is a diagram of an electric drive system with two separate powerconversion devices coupled in series applied in an EV system inaccordance with an exemplary embodiment of the present disclosure;

FIG. 6 is a flowchart of an energy management method for controllingpower flow in an electric drive system in accordance with an exemplaryembodiment of the present disclosure;

FIG. 7 is a flowchart of determining whether both of the first AC drivedevice and the second AC drive device are enabled shown in FIG. 6 inaccordance with an exemplary embodiment of the present disclosure;

FIG. 8 is a flowchart of calculating two torque control signals througha torque distribution unit shown in FIG. 6 in accordance with anexemplary embodiment of the present disclosure;

FIG. 9 is a graph of torque-speed waveforms of AC traction system usedin FIG. 8 in accordance with an exemplary embodiment of the presentdisclosure; and.

FIG. 10 is a flowchart of enabling at least one of the first energystorage unit and the second energy storage unit shown in FIG. 6 inaccordance with an exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

In an effort to provide a concise description of these embodiments, notall features of an actual implementation are described in one or morespecific embodiments. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers'specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

Unless defined otherwise, technical and scientific terms used hereinhave the same meaning as is commonly understood by one of ordinary skillin the art to which this disclosure belongs. The terms “first,”“second,” “third,” “fourth,” and the like, as used herein do not denoteany order, quantity, or importance, but rather are used to distinguishone element from another. Also, the terms “a” and “an” do not denote alimitation of quantity, but rather denote the presence of at least oneof the referenced items. The term “or” is meant to be inclusive and meaneither any, several, or all of the listed items. The use of “including,”“comprising,” or “having,” and variations thereof herein are meant toencompass the items listed thereafter and equivalents thereof as well asadditional items.

As used herein, the terms “may,” “can,” “may be,” and “can be” indicatea possibility of an occurrence within a set of circumstances; apossession of a specified property, characteristic or function; and/orqualify another verb by expressing one or more of an ability,capability, or possibility associated with the qualified verb.Accordingly, usage of “may,” “can,” “may be,” and “can be” indicate thata modified term is apparently appropriate, capable, or suitable for anindicated capacity, function, or usage, while taking into account thatin some circumstances, the modified tem may sometimes not beappropriate, capable, or suitable. For example, in some circumstances,an event or capacity may be expected, while in other circumstances, theevent or capacity may not occur. This distinction is captured by theterms “may,” “can,” “may be,” and “can be”.

FIG. 1 illustrates a block diagram of an electric drive system 10applied in a propulsion system (not shown) for driving two loads inaccordance with an exemplary embodiment of the present disclosure. Thepropulsion system may include a forklift system and a crane system, forexample. The electric drive system 10 includes an ESS 11, a powerconversion system 13, an AC traction system 15, and an energy managementsystem (EMS) 17. In some embodiments, the power conversion system 13 isconfigured for converting a DC power generated from the ESS 11 intoanother DC power provided to the AC traction system 15. In someembodiments the power conversion system 13 is configured for convertinga DC power generated from the AC traction system 15 into another DCpower for charging the ESS 11.

The EMS 17 is arranged to be in electrical communication with the ESS11, the power conversion system 13, and the AC drive system 15. In someembodiments, the EMS 17 may be configured to send power control signals171 to enable the power conversion system 13 to provide necessary DCpower for the AC traction system 15. In some embodiments, the EMS 17 maybe configured to send power control signals 171 to enable the powerconversion system 18 to provide necessary DC power for charging the ESS11. In some embodiments, the EMS 17 may be configured to send torquecontrol signals 173 to the AC traction system 15 according to one ormore command signals (e.g., electric drive system speed signal 25) toenable the AC traction system 15 to provide necessary mechanical torquesto the load system 31.

In some embodiments, the ESS 11 includes a first energy storage unit 12for providing or receiving a first power and a second energy storageunit 14 for providing or receiving a second power. In some embodiments,the ESS 11 may include more than two energy storage units. In someembodiments, the AC traction system 15 includes a first AC drive device20 for providing or receiving a first mechanical torque and a second ACdrive device 22 for providing or receiving a second mechanical torque.In some embodiments, the AC traction system 15 may include more than twoAC drive devices.

The power conversion system 13 includes a power conversion device 18.The power conversion device 18 includes a first terminal 24 electricallycoupled to the first energy storage unit 12, a second terminal 26electrically coupled to the second energy storage unit 14, a thirdterminal 28 electrically coupled to the first AC drive device 20, and afourth terminal 30 electrically coupled to the second AC drive device22. In some embodiments, the power conversion device 18 may include morethan four terminals.

When the electric drive system 10 is operated in an electric drivenmode, in some embodiments, the first terminal 24 may be configured toreceive a first input power from the first energy storage unit 12, andthe second terminal 26 may be configured to receive a second input powerfrom the second energy storage unit 14. In some embodiments, the powerconversion device 18 is configured to perform power conversion withrespect to the received first input power and second input power,provide a first output power at the third terminal 28, and provide asecond output power at the fourth terminal 30. The first output powermay be provided to the first AC drive device 20, and the second outputpower may be provided to the second AC drive device 22.

When the electric drive system 10 is operated in a regenerative orbraking mode, in some embodiments, the third terminal 28 may beconfigured to receive a first input power from the first AC drive device20, and the fourth terminal 30 may be configured to receive a secondinput power from the second AC drive device 22. In some embodiments, thepower conversion device 18 is configured to perform power conversionwith respect to the received first input power and the second inputpower, provide first output power at the first terminal 24 for chargingthe ESS 11 (e.g., first energy storage unit 12), and provide secondoutput power at the second terminal 26 for charging the ESS 11 (e.g.,second energy storage unit 14).

The power conversion system 13 may include a pre-charging switchingdevice 16 electrically coupled between the first energy storage unit 12and the second energy storage unit 14 for selectively providing a powerflow path between the first energy storage unit 12 and the second energystorage unit 14. In some embodiments, the power flow path may beunidirectional, so that the first energy storage unit 12 may deliverelectric power through the unidirectional power flow path for chargingthe second energy storage unit 14. In some embodiments, the power flowpath may be bi-directional, so that the electric power can be deliveredbetween the first energy storage unit 12 and the second energy storageunit 14 through the bi-directional power flow path.

A load system 31 may include a first load 27 coupled to the first ACdrive device 20 and a second load 29 coupled to the second AC drivedevice 22. In some embodiments, the first load 27 and the second load 29may receive a first mechanical torque from the first AC drive device 20and a second mechanical torque front the second AC drive device 22respectively. In some embodiments, when the electric drive system 10 isoperated in the regenerative or braking mode, the first load 27 and thesecond load 29 may provide a first mechanical torque and a secondmechanical torque to the first AC drive device 20 and the second ACdrive device 22 respectively. Then the first and second mechanicaltorques may be converted by the first and second AC drive devices 20,22, respectively, into electric power for charging the ESS 11.

With a combination of at least two energy storage units, at least onepower providing or receiving action may be taken in an appropriatemanner to improve the power utilization and extend the life of ESS. Witha combination of at least two AC drive devices, at least one powerconversion action may be taken in a flexible manner to improve theefficiency of the electric drive system.

FIG. 2 illustrates a block diagram of an electric drive system 50applied in a propulsion system (not shown) for driving a single load 23in accordance with an exemplary embodiment of the present disclosure.The propulsion system may include an EV system and an elevator system.The electric drive system 50 is similar to the electric drive system 10shown in FIG. 1 and includes the ESS 11, the power conversion system 13,the AC traction system 15, and the EMS 17. Thus, detail descriptionabout the ESS 11, the power conversion system 13, the AC traction system15, and the EMS 17 are omitted herein. In some embodiments, the electricdrive system 50 includes a power split device 19 mechanically coupledbetween the AC traction system 15 and the load 23.

The power split device 19 includes a first mechanical terminal 32, asecond mechanical terminal 34, and a third mechanical terminal 36. Thefirst mechanical terminal 32 is mechanically coupled to the first ACdrive device 20 for receiving or providing a first mechanical torque.The second mechanical terminal 34 is mechanically coupled to the secondAC drive device 22 for receiving or providing a second mechanicaltorque. The third mechanical terminal 36 is mechanically coupled to theload 23 for providing or receiving a third mechanical torque. In someembodiments, the power split device 19 includes one or more gears 35 fortransmitting mechanical torques provided by the AC traction system 15 tothe load 23.

In some embodiments, when the propulsion system is driving a heavy load,the first mechanical terminal 32 receives a first mechanical torque fromthe first AC drive device 20, the second mechanical terminal 34 receivesa second mechanical torque from the second AC drive device 22, and thethird mechanical terminal 36 provides a third mechanical torque to theload 23. The third mechanical torque is a combination of the firstmechanical torque and the second mechanical torque by the power splitdevice 19.

In some embodiments, when the propulsion system is operated in aregenerative or braking mode, the third mechanical terminal 36 receivesa third mechanical torque from the load 23, the first mechanicalterminal 32 provides a first mechanical torque to the first AC drivedevice 20, and the second mechanical terminal 34 provides a secondmechanical torque to the second AC, drive device 22. The firstmechanical torque and the second mechanical torque are split from thethird mechanical torque by the power split device 19. Then the firstmechanical torque and the second mechanical torque may be converted intoelectric power by the AC traction system 15 for charging the ESS 11.

In some embodiments, when the propulsion system is driving a light load,the first mechanical terminal 32 receives a first mechanical torque fromthe first AC drive device 20, the second mechanical terminal 34 providesa second mechanical torque to the second AC drive device 22, and thethird mechanical terminal 36 provides a third mechanical torque to theload 23. The second mechanical torque and the third mechanical torqueare split from the first mechanical torque by the power split device 19.Then the second mechanical torque may be converted into electric powerby the second AC drive device 22 for charging the ESS 11.

In some embodiments, the electric drive system 50 may include anoptional transmission device 21. The transmission device 21 may bemechanically coupled between the AC traction system 15 and the powersplit device 19 for matching the output speed of the AC traction system15 with the electric drive system speed.

In some embodiments, the transmission device 21 may be mechanicallycoupled between the first AC drive device 20 and the first mechanicalterminal 32 of the power split device 19. In some embodiments, thetransmission device 21 may be mechanically coupled between the second ACdrive device 22 and the second mechanical terminal 34 of the power splitdevice 19. In some embodiments, the transmission device 21 may beeliminated when the Output speed of the AC traction system 15 meetsrequirements of the propulsion system.

The weight, size, and cost of the electric drive system may be decreasedby selecting appropriate energy storage units, appropriate AC drivedevices, and an appropriate combination of the energy storage units andthe AC drive devices.

In some embodiments, the ESS 11 may include a battery and anotherbattery used as the first energy storage unit 12 and the second energystorage unit 14, respectively. In some embodiments, the ESS 11 mayinclude a battery and an ultra-capacitor used as the first energystorage unit 12 and the second energy storage unit 14, respectively. Insome embodiments, the ESS 11 may include other kinds of energy sourcesuch as a flywheel battery.

In some embodiments, the AC traction system 15 may include an inductionmotor (IM) and another IM used as the first AC drive device 20 and thesecond AC drive device 22, respectively. In some embodiments, the ACtraction system 15 may include an IM and an interior permanent motor(IPM) used as the first AC drive device 20 and the second AC drivedevice 22, respectively. In some embodiments, the AC traction system 15may include other kinds of AC motor.

FIG. 3 illustrates a diagram of an electric drive system 100 applied inan EV system (not shown) in accordance with an exemplary embodiment ofthe present disclosure. The electric drive system 100 includes an ESS102, a power conversion system 104, an AC traction system 106, an EMS108, a power split device 122, and a transmission device 119.

In some embodiments, the ESS 102 includes a battery 101 and anultra-capacitor 103. The AC traction system 106 includes a first ACdrive device 116 and a second AC drive device 118. The first AC drivedevice 116 includes a first DC/AC inverter 113 and an IM 115. The secondAC drive device 118 includes a second DC/AC inverter 113 and an IPM 117.In some embodiments, a single power conversion device 107 is used whichhas been described in detail in the power conversion device 18 shown inFIG. 1.

Several power flow paths may be formed in the electric drive system 100.In some embodiments, the battery 101 may be enabled to deliver electricpower through the power conversion system 104, which in turn providesconverted electric power to at least one of the IM 115 and the IPM 117,thereby at least a first power flow path is formed. In some embodiments,the ultra-capacitor 103 may be enabled to deliver electric power throughthe power conversion system 104, which in turn provides convertedelectric power to at least one of the IM 115 and the IPM 117, thereby atleast a second power flow path is formed.

In some embodiments, the mechanical torques of at least one of the IM115 and the IPM 117 firstly are converted into electric power and thenthe electric power may be delivered through the power conversion system104 for charging at least one of the battery 101 and the ultra-capacitor103, thereby at least a third power flow path is formed. In someembodiments, the battery 101 may deliver electric power through thepre-charging switching device 105 for charging the ultra-capacitor 103,thereby at least a fourth power path is formed.

A flexible charging strategy can be implemented to charge at least oneof the battery 101 and the ultra-capacitor 103. For example, in someembodiments, the ultra-capacitor 103 can be charged with electric powergenerated from the IM 115 and converted by the single power conversiondevice 107. This is different from the prior charging strategy thatcharging the ultra-capacitor 103 through the use of pre-chargingswitching device 105. In some embodiments, the EMS 108 may be configuredto implement a decoupling algorithm for controlling the IM 115 and theIPM 117 separately.

FIG. 4 illustrates a diagram of an electric drive system 200 with afirst power conversion device 207 and a second power conversion device209 coupled in parallel applied in an EV system (not shown) inaccordance with an exemplary embodiment of the present disclosure. Insome embodiments, the power split device 121 may include one or moreplanetary sets 37 for transmitting mechanical torques provided by the ACtraction system 106 to the load 123.

In some embodiments, the first power conversion device 207 includes afirst terminal 231 and a third terminal 235. The second power conversiondevice 209 includes a second terminal 233 and a fourth terminal 237.

In some embodiments, electric power from IM 115 may be delivered to theultra-capacitor 103 through the power conversion system 204 indirectlyvia the pre-charging switching device 105, and electric power from theIPM 117 may be delivered to the ultra-capacitor 103 through the powerconversion system 204.

A decoupling control of the two AC motors 115, 117 may be managed bycontrolling the first power conversion device 207 and the second powerconversion device 209 separately due to different output DC voltages atthe third terminal 235 and the fourth terminal 237.

Several charging paths may be implemented for charging theultra-capacitor 103. In some embodiments, the battery 101 may deliverelectric power through the pre-charging switching device 105 forcharging the ultra-capacitor 103. In some embodiments, the IM 115 maydeliver electric power through the first power conversion device 207 andthe pre-charging switching device 105 for charging the ultra-capacitor103. In some embodiments, the IPM 117 may deliver electric power throughthe second power conversion device 209 for charging the ultra-capacitor103. In some embodiments, at least a part of the mechanical torqueprovided from the IM 115 can be converted into electric power throughthe IPM 117 and the second power conversion device 209, so that theultra-capacitor 103 can be charged.

FIG. 5 illustrates a diagram of an electric drive system 300 with afirst power conversion device 307 and a second power conversion device309 coupled in series applied in an EV system (not shown) in accordancewith an exemplary embodiment of the present disclosure.

The first power conversion device 307 includes a first terminal 331, asecond terminal 333, and a third terminal 335. In some embodiments, thethird terminal 335 is electrically coupled to the second AC drive device118 and the second power conversion device 309. The second powerconversion device 309 includes an input terminal 337 electricallycoupled to the third terminal 335 of the first power conversion device307, and an output terminal 339 electrically coupled to the first ACdrive device 116. In some embodiments, the power flow paths may also beimplemented in a flexible manner similar to what has been described withreference to FIG. 3.

A decoupling control of the IM 115 and the IPM 117 may be achieved bycontrolling the first power conversion device 307 and the second powerconversion device 309 separately due to different output DC voltages atthe third terminal 335 and the output terminal 339.

In some embodiments, the second power conversion device 309 is coupledto the second AC drive device 118. In some embodiments, the powerconversion system 304 may include more separate power conversion devicesand a flexible manner in connecting these separate power conversiondevices.

FIG. 6 illustrates a flowchart of an energy management method 600 forcontrolling power flow in an electric drive system in accordance with anexemplary embodiment. At least some blocks of the method 600 may beprogrammed with software instructions stored in a computer-readablestorage medium which, when executed by a processor, perform varioussteps of the method 600. The computer-readable storage medium mayinclude volatile and nonvolatile, removable and non-removable mediaimplemented in any method or technology. The computer-readable storagemedium includes, but is not limited to, RAM, ROM, EEPROM, flash memoryor other memory technology, CD-ROM, digital versatile disks (DVD) orother optical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other non-transitorymedium which may be used to store the desired information and which maybe accessed by an instruction execution system.

In some embodiments, the method 600 may start at block 601, when ameasured electric drive system speed V_(speed) is received. Then theprocess goes to block 603, in some embodiments, a required power P_(req)is calculated by a total torque request signal T_(total) and V_(speed).Then the process goes to block 605.

At block 605, a determination is made to enable at least one of thefirst AC drive device 20 and the second AC drive device 22 based atleast in part on V_(speed). In some embodiments, the propulsion systemmay be operated in a normal mode based at least in part on V_(speed) Thenormal mode may include a starting mode, an accelerating or uphill mode,a cruising mode, and a regenerative or braking mode. In someembodiments, the propulsion system may be operated in a fault-toleratemode according to measured current signals or voltage signals.

In some embodiments, the operating AC drive device and an operatingmanner (e.g., motor, generator) of the AC motor in the AC drive deviceare determined based at least in part on the operating mode and P_(req).If both of the first AC drive device 20 and the second AC drive device22 are enabled, the process goes to 607, if not, the process goes to609.

At block 607, several optimization algorithms may be implemented in atorque distribution unit for distributing the total torque requestsignal T_(total) into a first torque command signal and a second torquecommand signal. The first torque command signal is provided to the firstAC drive device 20 and the second torque command signal is provided tothe second AC drive device 22. Then the process goes to block 609.

At block 609, a determination is made to enable at least one of thefirst energy storage unit 12 and the second energy storage unit 14 basedat least in part on the operating AC drive device and the structure ofthe power conversion system. In some embodiments, the operating manner(e.g., charger, discharger) of the first and second energy storage unitsis determined.

In some embodiments, a principle for determining the operating energystorage unit is that the first energy storage unit 12 is operated as aprimary power supplier, the second energy storage unit 14 is operated asan assistant. In some embodiments, the first energy storage unit 12provides power to the first AC drive device 20 and the second energystorage unit 14 provides power to the second AC drive device 22.

In some embodiments, the second energy storage unit 14 provides anadditional power when P_(req) rapidly that the first energy storage unit12 alone may fail to provide the required power P_(req) due to alimitation of its charge/discharge characteristic. In some embodiments,when the second energy storage unit 14 fails to provide the additionalpower, the first energy storage unit 12 can be relied on to provide theneeded power.

In some embodiments, the power provided by the first AC drive device 20and the second AC drive device 22 may be partly used to charge thesecond energy storage unit 14 when the propulsion system is operated ina regenerative or braking mode. Then the process goes to block 611.

At block 611, power control signals 171 based at least in part on theenabled AC drive device and the enabled energy storage unit isdetermined for controlling power converting process. The power controlsignals 171 may be obtained by the description below.

The electric power may be generated from the ESS 11 and provided to theAC traction system 15. In some embodiments, a first input power receivedby the first terminal 24 may be converted into a first output power atthe third terminal 28 and a second output power at the fourth terminal30.

In some embodiments, a second input power received by the secondterminal 26 may be converted into another first output power at thethird terminal 28 and another second output power at the fourth terminal30.

In some embodiments, a first input power received by the first terminal24 and a second input power received by the second terminal 26 areconverted into another first output power at the third terminal 28 andanother second power at the fourth terminal 30.

Then electric power received by the third terminal 28 and the fourthterminal 30 are selectively provided to the AC traction system 15 basedon the operating AC drive device.

The electric power may be generated from the AC traction system 15 andbe used to charge the ESS 11. In some embodiments, when the first ACmotor in the first AC drive device 20 is operated as a generator, theelectric power generated from the first AC drive device 20 may be usedto charge the ESS 11. The electric power can be delivered to the secondenergy storage unit 14 through the power conversion device 18 directlyor through the pre-charging device 16 via the power conversion device 18indirectly.

In some embodiments, when the second AC motor in the second AC drivedevice 22 is operated as a generator, the electric power generated fromthe second AC drive device 22 may be delivered to the ESS 11 through thepower conversion device 18 directly.

In some embodiments, when the first AC motor in the first AC drivedevice 20 is enabled as a motor and the second AC motor in the second ACdrive device 22 is enabled as a generator, the mechanical torque fromthe first AC drive device 20 may be split by the power split device 19into a first mechanical torque and a second mechanical torque. The firstmechanical torque is supplied to the load 23, and the second mechanicaltorque is converted to an electric power for charging the second storageunit 14 through the power conversion device 18.

In some embodiments, the power generated from the first energy storageunit 12 may be delivered to the second energy storage unit 14 throughthe pre-charging switching device 16. Then the process goes to block613. At block 613, the power control signals 171 is used to control thepower flow among the energy storage system 11, the power conversionsystem 13 and the AC traction system 15. Then the process goes to block615, torque control signals 173 are provided to the AC traction system15 for outputting desired torques for driving the load 23.

The process described above may be modified in a variety of ways. Insome embodiment, an additional block may be included before block 601,at the additional block, a determination is made to ascertain whether apre-charging process should be enabled according to a current, a voltageor a power P_(esu2) of the second storage unit 14. In some embodiments,when P_(esu2)<P₂ (where P₂ is a predetermined value which represents athreshold that the second energy storage unit needs to be charged), thepre-charging process is enabled. Otherwise, the pre-charging process isdisabled.

In some embodiments, the pre-charging process function is initiatedbefore the propulsion system is enabled. The pre-charging processincludes charging the second energy storage unit 14 from the firstenergy storage unit 12 through the pre-charging switching device 16which may ensure that the second energy storage unit 14 can provideenough power quickly when needed. Then the process goes to block 601.

FIG. 7 illustrates a flowchart of determining whether both of the firstAC drive derive and the second AC drive device are enabled shown in FIG.6 in an EV system in accordance with an exemplary embodiment of thepresent disclosure. The detailed illustration is based on the structureshown in FIG. 3. However, a person having ordinary skills in the art mayapply the method disclosed herein to other propulsion systems.

In some embodiments, the flowchart 700 May start at block 701, if the EVsystem fails, at least one of the measured current signals or thevoltage signals exceeds predetermined value, the EV system is operatedin a fault-tolerate mode as shown at block 705. Otherwise, the EV systemis operated in a normal mode as shown at block 703.

When the EV system is operated in the fault-tolerate mode, at least oneof the battery 101, ultra-capacitor 103, IM 115, and IPM 117 fails. Thenormal parts may be reconfigured as a simplified electric drive system.

In some embodiments, one of the battery 101 and ultra-capacitor 103 withboth of the IM 115 and IPM 117 are reconfigured. In some embodiments,both of the battery 101 and ultra-capacitor 103 with one of the IM 115and IPM 117 are reconfigured. In some embodiments, one of the battery101 and ultra-capacitor 103 with one of the IM 115 and IPM 117 arereconfigured. Therefore, the EV system may be allowed to stop in a safeway in the fault-tolerate mode with a fault-tolerant capability andreliability.

When the EV system is operated in the normal mode, after receiving avehicle speed v_(speed) 707 and calculating P_(req) with v_(speed) andT_(total) 709, the process goes to four branches 702, 704, 706, and 708.The four branches will determine the operating AC motor and theoperating manner. Index {1} is used to indicate that the IM 115 isenabled, index {2} is used to indicate that the IPM 117 is enabled, andindex {1, 2} is used to indicate that both of the IM 115 and IPM 117 areenabled.

At branch 702, the EV system is operated in a starting mode 711 whenV_(speed) increases from about 0. In the starting mode, a determinationis made to ascertain whether the IPM 117 can meet P_(req) 719, if yes,the process goes to block 723, in which the IPM 117 is enabled as amotor for providing necessary mechanical torque to meet P_(req). If notthe process goes to block 725, in which the IM 115 is enabled as a motoris enabled as a motor for providing necessary mechanical torque to meetP_(req).

At branch 704, the EV system is operated in an accelerating or uphillmode 713 when v_(speed) changes with a sudden increase over a period oftime. In the accelerating or uphill mode, the process goes to block 727,in which the IM 115 and IPM 117 are enabled as a motors for providingnecessary mechanical torques to meet P_(req).

At branch 706, the EV system is operated in a cruising mode 715 whenV_(speed) fluctuates within a scope of a small range. In the cruisingmode, the process goes to block 729, in which at least one of the IM 115and IPM 117 is enabled as a motor for providing necessary mechanicaltorque to meet P_(req).

At branch 708, the EV system is operated in a regenerative or brakingmode 717 when V_(speed) changes with a sudden decrease over a period oftime. In the regenerative or braking mode, the process goes to block731, in which both of the IM 115 and IPM 117 are enabled as generators.

FIG. 8 illustrates a flowchart of calculating two torque control signalsthrough a torque distribution unit shown in FIG. 6 in an EV system inaccordance with an exemplary embodiment of the present disclosure. Atblock 801, at least one signal such as T_(total) is received. At block803, a first AC motor torque signal referred to as “M_(torque1)” and asecond AC motor torque signal referred to as “M_(torque2)” are received.At block 805, a first AC motor speed signal referred to as “M_(speed1)”and a second AC motor speed signal referred to as “M_(speed2)” arereceived.

At block 807, a first torque command signal referred to herein as“T_(torque1)” and a second torque command signal referred to herein as“T_(torque2)” are calculated through a torque distribution unit. Thetorque distribution unit may be embodied as software which May beimplemented by the EMS 17 described above. In some embodiments,T_(total) may be provided by either the first AC motor (e.g., IM 115) orthe second AC Motor (e.g., IPM 117) or both the first and second ACmotors with an appropriate torque distribution algorithm.

In some embodiments, T_(torque1) and T_(torque2) may be calculatedthrough a fixed proportion algorithm which is based at least in part ontorque providing capability of the first and second AC motors as shownin FIG. 9. Waveforms 901 and 905 show the torque providing capability ofthe first and second AC motors (e.g., IM 115 and IPM 117). In anembodiment, one of the first and second AC motors can meet therequirement of T_(total), if not, the other AC motor will work toprovide an additional torque according to the following equations:

aT _(torque1) =T _(total)(0≦a≦1)  (1),

bT _(torque2) =T _(total)(0≦b≦1)  (2),

a+b=1  (3).

Where a and b are two variables, in some embodiments, a and b mayinclude a group of fixed values. When a=0, b=1, the first AC motor 115itself provides T_(total), when a=1, b=0, the second AC motor 117 itselfprovides T_(total). In some embodiments, a and b may include severalgroups of fixed values, so that an optimal torque distribution methodcan be implemented.

In some embodiments, T_(total) may be distributed to an average torquereferred herein as “T_(average)” and a peak torque referred herein as“T_(peak)”. T_(torque1) and T_(torque2) are equal to T_(average) andT_(peak) respectively. For example, a peak torque is needed, inparticular, with a sudden acceleration. In an embodiment, one of thefirst and second AC motors can provide the peak torque in a quick andstable response.

As shown in FIG. 9, the first AC motor can provide a large torque withina narrow speed range and the second AC motor can provide a smallertorque with a quick response within a wide speed range. In someembodiments, the second AC motor is used as an assistant which can workon and off. Basically, the algorithm is according to the followingequations:

T _(total) =T _(average) +T _(peak)  (4),

T_(torque1)=T_(average)  (5),

T_(torque2)=T_(peak)  (6).

In some embodiments, T_(torque1) and T_(torque2) may be calculatedthrough an algorithm of maximizing the system efficiency according tothe high efficiency areas 903, 907 of the first and second AC motorsshown in FIG. 9. In an embodiment, both the first and second AC motorscan work in their own high efficiency areas by using the second AC motorto assist the first AC motor. In some embodiments, the transmissiondevice 21 is used to change the operating point which can be implementedby changing M_(speed1) (e.g., waveforms 913, 915).

100901 In some embodiments, when M_(speed1) and M_(speed2) are fixedvalues, the operating points of the first and second AC motors may movealong with waveforms 909, 911 within the scope of the high efficiencyareas 903, 907. Thereby the total loss can be minimized The algorithmmay be determined according to the following equations:

T _(torque1) +T _(torque2) =T _(total)  (7),

P _(loss1) +P _(loss2)=min  (8).

Where P_(loss1) and P_(loss2) are power loss of the first and second ACmotors respectively, This method functions through checking a look uptable defining high efficiency areas of the two AC motors according toefficiency maps of each AC motor in the first AC drive device 20 and thesecond AC drive device 22 for reducing the total loss based at least inpart on the motor speeds M_(speed1) and M_(speed2).

FIG. 10 illustrates a flowchart of enabling at least one of the firstenergy storage unit and the second energy storage unit is enabled shownin FIG. 6 in accordance with an exemplary embodiment of the presentdisclosure, P_(req) is used to represent a required power, P_(bat) isused to represent a power provided by the battery 101, P_(uc) is used torepresent a power provided by the ultra capacitor 103,P_(bat(threshold)) is used to represent an upper limit power provided bythe battery 101, P_(uc(threshold)) is used to represent an upper limitpower provided by the ultra-capacitor 103, and P_(uc(charging)) is usedto represent a required charging power of the ultra-capacitor 103.

At block 1001, P_(req) and motor index are received. Then process goesto block 1003, a determination is made to ascertain which branch theprocess will go. The process goes to block 1005 when receiving index {1}or index {1, 2}. Otherwise, the process goes to block 1007 whenreceiving index {2}.

At block 1005, if P_(req) is larger than P_(bat), the process goes toblock 1009. Otherwise, the process goes to block 1011 in which thebattery 101 provides the power. At block 1009, if P_(uc(threshold)) islarger than an additional power P_(req)−P_(bat(threshold)), the processgoes to block 1013 that the battery 101 provides the upper limit poweritself and the ultra-capacitor 103 provides the additional power.

At block 1007, if P_(req) is larger than P_(u(threshold)), the processgoes to block 1017. Otherwise, the process goes to block 1019 that theultra-capacitor 103 provides the power. At block 1017, if theultra-capacitor 103 requires to be charged, the process goes to 1021that the ultra-capacitor 103 receives the required charging powerP_(uc(charging)), and the battery 101 provides both the required powerP_(req) and the required charging power of the ultra-capacitor 103P_(uc(charging)). Otherwise, the process goes to block 1023 that theultra-capacitor 103 provides the upper limit power itself and thebattery provides the additional power.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without depending fromthe spirit or essential characteristics thereof Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

1. An electric drive system, comprising: an energy storage system (ESS)comprising a plurality of energy storage units; an alternating current(AC) traction system coupled to the ESS, the AC traction systemcomprising a plurality of AC drive devices each configured to convertelectric power to mechanical torque; a power device coupled to the ACtraction system configured to combine the mechanical torque providedfrom the plurality of AC drive devices; and a load coupled to the powerdevice configured to receive the combined mechanical torque.
 2. Theelectric drive system of claim 1, further comprising an energymanagement system (EMS) in communication with the ESS and the ACtraction system.
 3. The electric drive system of claim 1, furthercomprising a power conversion system.
 4. The electric drive system ofclaim 3, wherein the plurality of energy storage units comprises a firstenergy storage unit and a second energy storage unit.
 5. The electricdrive system of claim 4, wherein the power conversion system furthercomprises: a first power conversion device coupled to the first energystorage unit; and a second power conversion device coupled to the secondenergy storage unit.
 6. The electric drive system of claim 5, whereinthe first energy storage unit comprises a battery and the second energystorage unit comprises an ultra-capacitor.
 7. The electric drive systemof claim 1, wherein the plurality of AC drive devices comprises: a firstAC drive device for providing or receiving a first mechanical torque;and a second AC drive device for providing or receiving a secondmechanical torque.
 8. The electric drive system of claim 7, wherein thepower device is further configured to combine the first mechanicaltorque and second mechanical torque into a third mechanical torque. 9.The electric drive system of claim 7, wherein: the first AC drive devicecomprises a first inverter coupled to a first electric motor; and thesecond AC drive device comprises a second inverter coupled to a secondelectric motor.
 10. The electric drive system of claim 9, wherein thefirst electric motor comprises an induction motor and the secondelectric motor comprises a permanent magnet motor.
 11. The electricdrive system of claim 1, further comprising a pre-charging switchingdevice.
 12. The electric drive system of claim 11, wherein thepre-charging switching device is a unidirectional switching device. 13.The electric drive system of claim 1, further comprising a transmissiondevice coupled between the AC traction system and the power device. 14.An electric drive system, comprising: a load configured to provide afirst mechanical torque; a power device coupled to the load configuredto split the first mechanical torque into a second mechanical torque andthird mechanical torque; an alternating current (M) traction systemcoupled between the power device and an energy storage system (ESS), theAC traction system comprising: a first AC drive device comprising afirst inverter coupled to a first electric machine, the first AC drivedevice configured to convert the second mechanical torque into electricpower for charging a first energy storage unit of the ESS; and a secondAC drive device comprising a second inverter coupled to a secondelectric machine, the second AC drive device configured to convert thethird mechanical. torque into electric power for charging a secondenergy storage unit of the ESS.
 15. The electric drive system of claim14, further comprising an energy management system (EMS) incommunication with the ESS and the AC traction system.
 16. The electricdrive system of claim 14, further comprising a power conversion system.17. The electric drive system of claim 14, wherein the first energystorage unit comprises a battery and the second energy storage unitcomprises an ultracapacitor.
 18. The electric drive system of claim 14,wherein the first electric machine comprises an induction motor and thesecond electric machine comprises a permanent magnet motor.
 19. Theelectric drive system of claim 14, further comprising a pre-chargingswitching device coupled between the first energy storage unit and thesecond energy storage unit.
 20. The electric drive system of claim 14,further comprising a transmission device coupled between the AC tractionsystem and the power device.